1. Field of the Invention
This invention generally relates to heat exchangers, such as large scale industrial boilers, and particularly relates to a method for removing deposits and encrustations from the heat transfer surfaces of the heat exchanger components.
2. Description of the Related Art
Trash burners and large scale boilers used by public utilities and industry are often fired by ash producing fossil fuels. As a result of these fuels, the internal surfaces of the boiler often become fouled with encrustations of soot, slag and ash during use. To optimize the thermal efficiency of the boiler, it is necessary to periodically remove and clean these deposits from the heat exchanger surfaces.
One type of system presently in wide spread use for deslagging the heat exchanger surfaces is known as a sootblower. Sootblowers are generally mounted exteriorly of the boiler and include a lance tube with a nozzle at one end. The lance is periodically inserted into the boiler through ports located in the boiler wall. While within the boiler, a cleaning medium, such as water, steam, air or another solution, is projected under pressure from the nozzle to impinge against the heat exchanger's surfaces. The mechanical impact and thermal shock caused by impingement of the cleaning medium results in the fracturing and dislodgement of the encrustations from the heat exchange surfaces.
While sootblowers generally operate satisfactorily and are superior devices in many boiler applications, they have certain limitations. First, sootblowers tend to consume a significant amount of the cleaning medium. This is a direct expense which must be recovered by the operator of the heat exchange facility. Additionally, sootblowers are unable to reach areas of the heat exchanger which are inaccessible or beyond the effective discharge range of the cleaning medium. Sootblowers also tend to clean the heat exchange surfaces down to the bare metal. In the highly corrosive and acidic environment found within the boiler, this leaves the cleaned heat exchange surface readily susceptible to corrosion.
Another system for removing encrustations from the heat exchanger surfaces utilizes what are known as rappers. Rappers typically have an impacter which, through a mechanical linkage, produces mechanical vibration in the heat exchange surface. The vibration causes the deposits to disintegrate, fracture and/or dislodge from the heat exchanger surfaces. One advantage of the mechanical rapper has over a sootblower is that the mechanical rapper does not remove the protective oxide layer on the heat exchanger surface. This left behind oxide layer helps to protect the heat exchange surfaces from the corrosive boiler environment. Typically, rappers are manually, pneumatically or electrically actuated.
The most common form of electrically actuated rappers are the falling hammer rapper. In this approach, fixed anvil-shaped weights (hammers) are lifted by a mechanical linkage which is connected to a rotating shaft that is driven by an electric motor. The hammers are allowed to fall and strike an impact transfer pin which imparts mechanical vibration to the heat exchange surfaces and dislodges the encrustations.
Another approach of providing an electrically actuated rapper is to use a solenoid to propel an impacter forward against an impact target. These rappers typically use a spring to retract the impacter after it has been mechanically actuated to impact its associated mechanical linkage during the rapping sequence. Such springs, however, are a drawback since they mechanically wear out relatively quickly and since they introduce vibration into the mechanical impacter itself.
Another approach for providing an electrically actuated rapper uses a dual coil, electromagnetic hammer rapper. The coils are separately energizable to cause forward or retracted movements of a impacter or armature. When the forward coil is energized, the impacter is moved forward to strike an impact transfer pin causing the kinetic energy to be transferred from the impacter and transfer pin to the heat exchange surfaces whereby it induces mechanical vibration and deslags the heat exchange surfaces. When the retraction coil is energized, the impacter is propelled in the rearward direction out of engagement with the impact transfer pin.
Another drawback with the above mechanical and electromechanical rappers is the need for a tie bar affixed to the heat exchangers surfaces and extending up to and through the boiler wall. Tie bars represent the primary difficulties and highest expense associated with any of the above mentioned types of rappers. At the boiler wall, a wall box and sleeve assembly is required to permit passing of the impact transfer pin through the boiler wall to the point where it can be impacted by the rapper device. Additionally, the mechanical tie bar which interconnects the heat exchanger surfaces must be positioned and aligned so that it receives the energy being transmitted through the impact transfer pin. If the boiler was not originally designed to accommodate rappers and the tie bars, the installation of the tie bars requires substantial modification to the boiler. Furthermore, existing tie bar materials and designs limit the use of rapper arrangements to the low temperature regions of the boiler, typically those regions below 1600.degree. F., or where the use of continuous active cooling, such as by recirculating water or air through the tie bar. This need for continual cooling, however, makes the device uneconomical in most applications.
It is therefore a principal object of this invention to provide an apparatus and method by which encrustations and other deposits can be removed from the heat exchange surfaces without requiring the use of sootblowers or mechanical rappers and tie bar assemblies.
It is a further object of this invention to provide an apparatus and method for removing encrustations and deposits from the heat exchange surfaces in a boiler through the inducement of mechanical vibration in the surfaces themselves.
It is another object of the present invention to provide a mechanism for cleaning the heat exchange surfaces by producing controlled pressure pulsations of a given magnitude within the heat transfer elements of at predetermined locations. The boiler thereby induces a vibration in the exterior of the heat exchange surfaces causing dislodgement of the encrustation and deposits thereon.
Still another object of this invention is to provide a mechanism by which steam can be injected into subcooled liquid zones of the boiler thereby inducing a vibration in the heat exchange surfaces causing removal of encrustations and deposits thereon.
It is another object of the present invention to provide an apparatus and method which can be used to clean heat transfer surfaces in the high temperature regions of the boiler, including those regions above 1600.degree. F.